Subsea fluid storage unit

Abstract

A modular subsea fluid storage unit has a variable-volume inner tank having a rigid top panel and a peripheral wall that is flexible by virtue of concertina formations. The peripheral wall is extensible and retractable vertically while the horizontal width of the tank remains substantially unchanged. A side wall of a lower housing part surrounds and is spaced horizontally from the peripheral wall of the inner tank to define a floodable gap between the peripheral wall and the side wall that surrounds the tank. An upper housing part extends over and is vertically spaced from the top panel of the inner tank and overlaps the side wall to enclose the inner tank. The floodable gap and the upper housing part enhance thermal insulation and trap any fluids that may leak from the inner tank.

The invention claimed is: 
     
       1. A subsea fluid storage unit, comprising:
 a variable-volume inner tank having a rigid top panel and a peripheral wall that is extensible and retractable in a vertical direction; 
 a side wall that surrounds and is spaced from the peripheral wall of the inner tank in a horizontal direction to define a floodable gap that surrounds the tank between the peripheral wall and the side wall; 
 an upper housing part that extends over and is spaced from the rigid top panel of the inner tank in the vertical direction and cooperates with the side wall to enclose the inner tank; and 
 a seawater inlet/outlet communicating with the floodable gap; 
 wherein the upper housing part overlaps the side wall in the horizontal direction and wherein a skirt of the upper housing part extends substantially parallel to the side wall on an external side of the side wall such that the side wall is received telescopically within the skirt, and wherein the rigid top panel is sandwiched between the upper housing part and the side wall. 
 
     
     
       2. The unit of  claim 1 , wherein the upper housing part overlaps the inner tank in the horizontal direction. 
     
     
       3. The unit of  claim 1 , wherein the upper housing part is substantially continuous across a full width of the inner tank in the horizontal direction. 
     
     
       4. The unit of  claim 1 , wherein the skirt adjoins a ledge that projects outwardly from the side wall. 
     
     
       5. The unit of  claim 1 , wherein the upper housing part rises inwardly from the side wall to an elevated fluid trap chamber for trapping fluid leaked from the inner tank. 
     
     
       6. The unit of  claim 5 , further comprising a leakage sensor arranged to sense leaked fluid in the fluid trap chamber and a drainage pipe that communicates with the fluid trap chamber to drain the leaked fluid. 
     
     
       7. The unit of  claim 1 , wherein the inner tank is closed by a bottom plate that extends in the horizontal direction along a bottom edge of the peripheral wall. 
     
     
       8. The unit of  claim 7 , wherein the bottom plate projects beyond the peripheral wall in the horizontal direction. 
     
     
       9. The unit of  claim 8 , wherein the clearance between the bottom plate and the side wall is a sliding clearance. 
     
     
       10. The unit of  claim 7 , wherein the bottom plate supports a heating system for heating fluid contents of the tank in use. 
     
     
       11. The unit of  claim 1 , wherein the side wall is thermally insulated. 
     
     
       12. The unit of  claim 1 , wherein the side wall has lower thermal transmittance than the peripheral wall of the inner tank. 
     
     
       13. The unit of  claim 1 , wherein the peripheral wall is flexible. 
     
     
       14. The unit of  claim 13 , wherein the peripheral wall has greater stiffness in the horizontal direction than in the vertical direction. 
     
     
       15. The unit of  claim 13 , wherein the peripheral wall comprises folded or hinged formations that are expandable in the manner of a concertina. 
     
     
       16. The unit of  claim 1 , wherein the side wall is contiguous with a base to define a floodable enclosure extending beneath the inner tank. 
     
     
       17. The unit of  claim 16 , wherein the floodable enclosure communicates with the floodable gap. 
     
     
       18. The unit of  claim 16 , wherein the base is thermally insulated. 
     
     
       19. The unit of  claim 16 , further comprising a seawater inlet/outlet communicating with the enclosure. 
     
     
       20. The unit of  claim 16 , wherein the inner tank closes an open top of the enclosure. 
     
     
       21. The unit of  claim 20 , wherein the rigid top panel of the inner tank is supported by the side wall. 
     
     
       22. The unit of  claim 21 , wherein the side wall supports a hanging flange of the top panel that projects beyond the peripheral wall in the horizontal direction. 
     
     
       23. The unit of  claim 1 , further comprising a leakage sensor arranged to sense fluid in the floodable gap leaked from the inner tank and a drainage line that communicates with the floodable gap to drain the leaked fluid. 
     
     
       24. The unit of  claim 1 , wherein the rigid top panel of the inner tank rises inwardly from the side wall to an elevated gas trap chamber for trapping gas rising from a fluid in the inner tank. 
     
     
       25. The unit of  claim 1 , comprising subsea-releasable fastenings acting in tension between the inner tank and the side wall. 
     
     
       26. The unit of  claim 1 , wherein the side wall has at least a portion that is substantially flat. 
     
     
       27. A group of units of  claim 1 , coupled together for fluid communication between the inner tanks of the group. 
     
     
       28. The group of  claim 27 , wherein the units are arranged in an elongate towable array.